Lithium titanium oxide having a spinel structure (Li4Ti5O12), which is in actual use, has three lithium ions capable of absorbing and releasing per unit chemical formula. For this reason, the number of lithium ions capable of absorbing and releasing is ⅗ per titanium ion; that is, 0.6 is theoretically the maximum value. Such a lithium titanium oxide having a spinel structure has a theoretical capacity of about 170 mAh/g.
Recently, titanium oxide having a monoclinic β-type structure has been drawing attention as a negative electrode material having a high capacity. In the titanium oxide having a monoclinic β-type structure, the number of lithium ions capable of absorbing and releasing is 1.0 at most per titanium ion. The titanium oxide, accordingly, has a high theoretical capacity of about 330 mAh/g, and a reversible capacity of about 240 mAh/g.
When the titanium oxide having a monoclinic β-type structure is used alone as the negative electrode active material, however, adhesion between a negative electrode active material-including layer and a current collector is reduced, and thus the negative electrode active material-including layer is separated from the current collector and excellent cycle characteristics cannot be obtained.